The selenium protein required for glycine reduction in Clostridium sticklandii has been characterized with regard to its fundamental physical and chemical properties. The selenium-containing moiety was identified as selenocysteine based upon the isolation of Se-aminoethyl-selenocysteine from the protein previously alkylated with ethylene imine. Purification of the additional enzymatic components of the glycine reductase system is being pursued in order to understand the function of the selenium protein in the overall reaction.